High temperature corrosion-resistant austenitic steel

ABSTRACT

THIS PATENT RELATES TO CORROSION-RESISTANT AUSTENITIC STEELS POSSESSING IMPROVED MECHANICAL AND CHEMICAL RESISTANCE PROPERTIES AT HIGH TEMPERATURES. THE INVENTION IS MORE SPECIALLY THOUGH NOT EXCULUSIVELY DIRECTED TO SUCH STEELS HAVING IMPROVED CORROSION-RESISTANT CHARACTERISTICS IN THE PRESENCE OF NOT ONLY LEAD OXIDE BUT ALSO VANADIUM PENTOXIDE AND WHICH WILL CONSEQUENTLY BE HIGHLY SUITABLE FOR USE IN THE CONSTRUCTION OF CERTAIN PARTS OF INTERNAL COMBUSTION ENGINES, INCLUDING ESPECIALLY THE EXHAUST VALVES THEREOF, WHICH ARE LIABLE TO BE EXPOSED TO THE COMBUSTION PRODUCTS OF LEAD TETRAETHYL AS PRESENT IN HIGH GRADE PRESENT-DAY FUELS OR LOW GRADE RESIDUAL FUEL OILS FOR THE OPERATION OF PRESENT-DAY DIESEL ENGINES. THIS INVENTION DISCLOSES THAT THE ABOVE MENTIONED DESIRABLE RESULTS ARE OBTAINED INCERTAIN CHROMIUM-CONTAINING AUSTENITIC STEELS BY USING COMPOSITION THAT EXHIBIT THE FOLLOWING FEATURES: (1) THE USE OF A CARBON CONTENT WITHIN A RESTRICTED RANGE OF ABOUT 0.30 TO 0.70%, (2) THE USE OF SILICON CONTENTS OF 0.10 TO 1.0%, (3) MANGANESE OF 5.0 TO 12.0%, (4) NICKEL OF 2.0 TO 7.0%, (5) CHROMIUM OF 18.0 TO 25.0%, (6) NITROGEN OF 0.20 TO 0.60%, (7) ARSENIC OF 0.02 TO 1.0%, (8) THE USE OF BORON IN AMOUNTS UP TO 0.1%, ALL BY WEIGHT, THE BALANCE BEING SUBSTANTIALLY IRON WITH INCIDENTAL IMPURITIES.

United States Patent Ofice Int. (:1. Che 39/20 US. Cl. 75-128 6 Claims ABSTRACT OF THE DISCLOSURE This patent relates to corrosion-resistant austenitic steels possessing improved mechanical" and chemical resistance properties at high temperatures. The invention is more specially though not exclusively directed to such steels having improved corrosion-resistant characteristics in the presence of not only lead oxide but also vanadium pentoxide and which will consequently be highly suitable for use in the construction of certain parts of internal combustion engines, including especially the exhaust valves thereof, which are liable'to be exposed to the combustion products of lead tetraethyl as present in high grade present-day fuels or low grade residual fuel oils for the operation of present-day diesel engines.

This invention discloses that the above mentioned desirable results are obtained in certain chromium-containing austenitic steels by using composition that exhibit the following features: (1) the use of a carbon content within a restricted range of about 0.30 to 0.70%, (2) the use of silicon contents of 0.10 to 1.0%, (3) manganese of 5.0 to 12.0%, (4) nickel of 2.0 to 7.0%, (5) chromium of 18.0 to 25.0%, (6) nitrogen of 0.20 to 0.60%, (7) arsenic of 0.02 to 1.0%, (8) the use of boron in amounts up to 0.1%, (9) the use of sulfur in substantial amounts up to 0.1%, all by weight, the balance being substantially iron with incidental impurities.

This invention relates to corrosion-resistant austenitic steels possessing improved mechanical and chemical resistance properties at high temperatures. The invention is more specially though not exclusively directed to such steels having improved corrosion-resistant characteristics in the presence of not only lead oxide but also vanadium pentoxide and which will consequently be highly suitable for use in the construction of certain parts of internal combustion engines, including especially the exhaust valves thereof, which are liable to be exposed to the combustion products of lead tetraethyl as present in high grade present-day fuels or low grade residual fuel oils for the operation of present-day diesel engines.

This invention discloses that the above mentioned desirable results are obtained in certain chromium-containing austenitic steels by using composition that exhibit the [following features: (1) the use of a carbon content within a restricted range of about 0.30 to 0.70%, (2) the use of silicon contents of 0.10 to 1.0%, (3) manganese of 5.0 to 12.0%, (4) nickel of 2.0 to 7.0%, (5) chromium of 18.0 to 25.0%, (6) nitrogen of 0.20 to 0.60%, (7) arsenic of 0.02 to 1.0%, all by weight, the balance being substantially iron with incidental impurities.

In recent years there have been developed grades of austenitic stainless steel having good corrosion-resistant properties at high temperatures, and characterized in that they include, in addition to iron, carbon, a major proportion of chromium, some manganese and nickel, a small proportion of nitrogen as well as silicon.

One of the commercially available valve compositions 3,554,736 Patented Jan. 12, 1971 in present use today in an austenitic alloy, referred to the steel as 21-4N, which possesses a nominal C0111- mercial analysis of about 0.6% carbon, about 0.25% silicon, about 9% manganese, about 4% nickel, about 21% chromium, about 0.4% nitrogen and the balance iron. This steel has good corrosion-resistant properties in the presence of lead oxide at high temperatures.

Marine diesel engines may be made to burn a variety of fuels, for example, bunker fuel, marine fuel and residual fuel oil, thus providing a cheap source of power. It has been found that under the severe operating condition of marine diesel engines 21-4N steel are inclined to corrode much too rapidly when the combustion products of vanadium oxide by the use of residual fuel oils containing vanadium are encountered at high temperatures.

The experimental work underlying this invention has brought to light the unexpected fact that the addition of much arsenic into the steel, brings about a marked decrease in the corrosion rate of the steel at high temperatures, in vanadium pentoxide attack.

An improved steel composition according to the invention may comprise the following formulation by weight, in addition to iron and the usual impurities in the usually accepted ranges:

Percent Carbon 0.30-0.70 Silicon 0.10-4.0 Manganese 5.0-12.0: Nickel 2.0-7.0 Chromium 18.0-25.0 Nitrogen 0.20-0.60 Arsenic 0.02-1.0 Boron Up to 0.1 Sulfur Up to 0.1

Carbon is elfective for forming carbides with chrornium. It has been found that at least 0.3% carbon is necessary in order to obtain the stable austenite, requisite, strength and hardness. Carbon contents in excess of about 0.70% adversely affect the machinability and contribute to reducing the hot workability and ductility of the alloy. Optimum results appear to be obtained when the carbon content is maintained with the range between 0.30% and 0 .70%.

Silicon contents in excess of 1.0% appear to adversely affect the resistance to lead oxide of the alloy. Optimum results are obtained when the silicon content is maintained within the range between 0.10% and 1.0%.

Manganese usually contains between 0.5 and 2% for deoxidizer. Manganese materially contributes to the resistance to lead oxide and at least 5% is necessary. Manganese contents in excess of about 12%, while being effective for increasing the austenitic stability as well as nickel, adversely affect the resistance to oxidizing. Optimum results appear to 'be obtained where the manganese content is maintained within the range between 5.0% and 12.0%.

As respects nickel, the steel must contain an amount sufficient to insure that is required for good austenitic stability, good ductility and good resistance to lead oxide and vanadium pentoxide attack. The use of amounts of nickel greater than those indicated above is Without substantial benefit justifying the addition and is contraindicated on account of cost. Optimum results are obtained when the nickel content is maintained within the range between 2.0% and 7.0%, because the nickel will possess twice as large as austenitic stability of the manganese.

Nitrogen is highly critical and is present within the alloy as an interstitial hardener. Thus it has a great influence on the strain hardening rate of the alloy. At least 0.2% nitrogen is necessary within the alloy in order to insure the high temperature strength and austenite stability. Increasing the nitrogen content to above about 0.60% results adversely affecting the hot workability, and also decreases the resistance to vanadium pentoxide of this alloy. Optimum results appear to be obtained when the nitrogen content is maintained within the range of about 0.20% and 0.60%.

Chromium is present within the range between 18% and 25%. Within this range the alloy possesses the required degree of corrosion resistance, especially in atmospheres containing combustion products of leaded and containing vanadium fuels. In addition, at least 18% chromium is necessary in order to afford the alloy an acceptable measure of resistance to oxidation at elevated temperatures, but the use of an amount too great must be avoided in order to prevent the decrease of hot workability. The most satisfactory combination of properties occurs when the chromium content is within the range between 18.0% and 25.0%.

It has been discovered, by this inventor, that addition of arsenic brings about a marked decrease in the corrosion rate of the steel in atmospheres containing vanadium pentoxide. At least 0.02% arsenic is necessary within the alloy in order to insure these properties. Amount of arsenic beyond 1.0% tends to worsen hot workability and resistance of the steel to corrosion by lead oxide, 50 that it is preferable that the arsenic content is maintained within the range of about 0.02% and 1.0%.

As respects boron, the use of this element is optional,

but to provide better creep strength, it is desirable to include an effective addition of boron. Amounts of boron up to 0.1% have a favorable effect upon creep strength, but boron additions of more than about 0.04% tend to worsen resistance of the steel to high temperature corrosion, so that it is preferable that the boron not ex- TABLE l.CIIE1\1ICAL COMPOSITION OF STEELS TESTED Chemical composition, percent Si Mn S Ni Cr Ni As B Steel N o A 0.57 0. 9. 1 0.051 4. 1 21.1 0. 0. 008 0.001 B 0. 59 0. 00 9. 2 0.031 3. 0 21. 2 0. 48 0. 08 0. 005 C 0.58 0.50 0 2 0.033 4.0 21.0 0.43 0.21 0.006 D 0. 0.49 9 3 0. 03 4.1 21.0 0.41 0.45 0.000

The steel, designated A in this table is equivalent to 2l4N steel, while B, C and D are the steels of the present invention. These steels were aged for 6 hours at 750 C. after solution treated at 1150 C. Table 2 shows the results of corrosion tests by lead oxide at 916 C. The steels invented correspond to 214N steel in resistance to corrosion by lead oxide.

TABLE 2.RESULTS OF CORROSION TEST BY LEAD OXIDE AT 916 C.

Steel No.: Weight loss (gr./dm. /hr.) A 19.5 B 19.8 C l8.3 D 19.3

Table 3 shows the results of vanadium attack tests by melted vanadium pentoxide containing 10% sodium sulfate at 900 C. The Steels Of our invention are considerably superior in its resistance to corrosion than 21-4N.

TABLE 3. RESULTS OF VANADIUM ATTACK TEST AT 900 C.

Steel No.: Weight loss (gr./dm. /hr.) A 25.0 B 19.0

Table 4 also shows the hardness aged for 6 hours at 750 C. after solution treated at 1150 C.

TABLE 4.AGED HARDNESS Steel No.2 Hardness (HRC) A 32.5 B 32.0 C 32.3 D 32.0

The hardness at room temperature and elevated temperatures that shows in these steels are similar to 2l4N steel. The results of tensile test at room and elevated temperatures are given in Table 5.

TABLE 5.TENSILE TEST RESULTS TABLE 6.IMPACT STRENGTH AT 800 C.

Steel No.2 Charpy impact value (kgm./cm.

A 4.9 B 4.3 C 4.3 D 4.3

Table 6 also shows the Charpy impact test at 800 C. for each of the steels. The steel of the present invention has the strength and ductility that shows in each case with the 21-4N steel.

As many described, this invented steel that is added much arsenic than incidental impurities to 214N, is characterized by exhibiting excellent resistance to not only vanadium pentoxide attack but lead oxide attack at elevated temperatures. This steel is suitable for use in diesel engine valves, valve parts and other internal combustion engine components because it has also a strength at room temperature and high temperatures.

What is claimed is:

1. An austenitic steel composition consisting essentially of substantially 0.30-0.70% carbon, 0.11.0% silicon, 5.0l2.0% manganese, 2.0-7.0% nickel, 18.0- 25.0% chromium, 0.200.60% nitrogen, 0.02-1.0% arsenic, up to 0.1% boron and up to 0.1% sulfur, all by weight, the balance being substantially iron, and characterized by its high corrosion resistance properties in the presence of lead oxide and vanadium pentoxide, at high temperatures.

2. An austenitic steel composition consisting essentially of substantially 0.30-0.70% carbon, 0.l-l.0% silicon, 5.0l2.0% manganese, 2.07.0% nickel, 18.0 25.0% chromium, 0.200.60% nitrogen and 0.02-1.0% arsenic, all by weight, the balance being substantially iron, and characterized by its high corrosion resistance properties in the presence of lead oxide and vanadium pentoxide, at high temperatures.

3. A corrosion-resistant composition of austenitic steel consisting essentially of, by Weight, substantially 0.300.70% carbon, 0.ll.0% silicon, 5.0l2.0% manganese, 2.07.0% nickel, l8.0-25.0% chromium, 0.20- 0.60% nitrogen and 0.02-1.0% arsenic, the balance being substantially iron.

4. High temperature corrosion-resistant austenitic steel composition consisting essentially of substantially 0.40"- 0.60% carbon, 0.4-0.6% silicon, 8.*010.0% manganese, 4.05.0% nickel, 20.0-22.0% chromium, 0.3'0.5% nitrogen and 01-03% arsenic by weight, the balance being substantially iron.

5. Cast article for use as a part of an internal combustion engine and composed of a high-temperature corrosion-resistant austenitic steel consisting essentially of substantially 0.30- 0.70% carbon, 0.1-1.0% silicon, 5.0- 12.0% manganese, 2.'07.0% nickel, 18.0-25.0% chromium, 0.20-'0.60% nitrogen and 0.021.0% arsenic, all by Weight, the balance being substantially iron.

6. Exhaust valve for use in an internal combustion engine, composed of a high-temperature corrosion-resistant austenitio steel consisting essentially of substantially 0.30-0.70% carbon, (ll-1.0% silicon, 5.0-12.0% manganese, 2.07.0% nickel, l8.025.0% chromium, 0.20- 0.60% nitrogen and 0.02-1.0% arsenic, all by weight, the balance being substantially iron.

References Cited HYLAND BIZOT, Primary Examiner 

